It is common for certain classes of test equipment to have input impedances determined by precision resistors located within the instrument, or piece of such test equipment. For example, a spectrum analyzer or network analyzer may have its input impedance determined principally by a fifty or seventy-five ohm termination resistance at the entrance of an attenuator shunting the input of an amplifier or mixer. Also, high frequency oscilloscopes often have fifty ohm inputs to properly terminate signals conducted to the instrument via transmission lines of fifty ohm characteristic impedance. In modern equipment of this type it is common for the termination resistance to be a physically small device that is part of a hybrid circuit fabricated upon a substrate. This means that the input termination resistor cannot be allowed to dissipate high power, and that the resistor might not be individually replaceable if it burns out, even if attempted at the factory level.
Even the lowly AC voltmeter sometimes is configured to present a specific resistive impedance of, say, six hundred ohms, to a source being measured. For specialized meters used exclusively in a particular environment the normally desirable high impedance of the input is shunted internally with a termination resistor, the better to cooperate with attenuators calibrated in db and designed for use in a system of a particular impedance. The issue in these instances is not so much the avoidance of the reflection of power, which is the case in the termination of coaxial or waveguide transmission lines, but the (low frequency) "voltage divider" properties of a load resistance upon the internal impedance of a source.
In many instances the input of such a voltmeter will be AC coupled, thus protecting the input from the accidental application of DC voltage that might burn out the termination resistor. However, it is still possible that high AC voltages (which are indeed available at low frequencies) could fry an input not intended for or protected against high voltages. Furthermore, if the meter is an RMS meter with a low input impedance, then it may not be AC coupled at all, making it additionally vulnerable to damage from high DC potentials.
Oscilloscopes and voltmeters that have high input resistances generally do not have a burn-out problem arising from excessive power dissipation; one or ten megohms simply doesn't carry enough current for the dissipated power to rise to dangerous levels for the voltage levels normally apt to be encountered. (Medium and very high voltage circuits and their components do have their own failure mechanisms, but overheating due to excessive power dissipation is not high on the list.) However, such oscilloscopes and voltmeters with high impedance inputs are frequently converted for use in fifty, seventy-five or six hundred ohm systems, through the use of a feed-through ("feed-thru") termination, which is nothing more than a shunt resistance (often located in a barrel connector) attached to the input terminals of the instrument. Clearly, the feed-through termination is vulnerable to accidental destruction. An operator can easily forget that it is there, and move the probe from a low level AC signal over to a power supply of say, twenty or thirty volts. By the time the operator realizes his mistake the termination resistor can be reduced to so much charcoal and a certain distinctive odor that lingers over the workbench. Even a two watt fifty ohm resistor dissipates rated power at a mere ten volts; at twenty volts it would be four times as much, or eight watts.
Fuses have sometimes been used to protect termination resistors of the sort we have been discussing. Fusing works in low frequency situations, although it is annoying to discover that a spare fuse is not at hand when needed. Fusing is generally an unfavorable solution for high frequency situations, owing to the disturbance it creates in the impedance of the system.
It would be desirable if a termination resistance, whether for high or low frequencies, and whether internal or feed-through, could be protected against accidental burn-out. This can be achieved by placing a temperature sensitive resistive device of positive temperature coefficient (PTC) in series with the termination resistor, and thermally coupling that PTC device to the termination resistor. The preferred PTC device is a type of thermistor called a "PTC switch" that has a very low resistance below a certain critical temperature (say, a few tenths of an ohm below about one hundred and twenty degrees centigrade) and a resistance several orders of magnitude greater above the critical temperature. The critical temperature may also be the same as, or be closely related to, or be dependent upon, the Curie temperature of the material of which the PTC switch is composed. By thermally coupling the two together, the temperature rise in the termination resistor that accompanies excessive power dissipation causes the PTC switch to "trip". The increased total resistance markedly reduces the power dissipation to safe levels, even for fairly high levels of applied voltage. The desired protection can be obtained by the simple expedient of mounting the termination resistor and the PTC switch in close physical proximity to one another. This not only helps reduce unwanted reactances in the interconnecting conductors, but allows adequate thermal coupling. A more sophisticated and general solution is to fabricate a unit assembly comprising the series combination of the termination resistor and the PTC switch. This allows much better control over stray reactances, convenient mounting and very close thermal coupling between the PTC switch and the resistor. The unit assembly can then be treated as if it were a "smart" termination resistor. The form factor of such a unit assembly could be unique, so as to be readily identifiable or to facilitate the fabrication thereof, or it could be chosen to match that of the resistor that would otherwise be used: surface mount unit assemblies would resemble surface mount resistors, pellet resistors could be replaced by pellet unit assemblies, and unit assemblies in axial lead packages would be usable in place of standard axial lead resistors.
The particular location within a circuit that such a PTC switch/resistor combination is deployed will depend upon the nature of that specific circuit, and how the surrounding components react to the application of high voltage and the subsequent increase in resistance of the protected resistor. In oscilloscopes, for example, the internal attenuator is commonly a high impedance compensated voltage divider, and the preamplifier connected thereto is built to withstand the application of high voltages, even in the most sensitive settings of the attenuator. In these cases the fifty ohm termination is a switchable resistance placed between ground and the input to the compensated attenuator. Since the protected termination resistor is in parallel with the rest of the input circuitry, raising the resistance of that termination resistor does not prevent the applied high voltage from reaching further into the circuit. In this setting the fifty ohm termination is the only part needing protection, and the preamplifier is not harmed by continuing to allow the high voltage to reach it. In other settings, such as constant impedance attenuators, the protected resistor may need to be placed in series so that it may additionally protect subsequent circuitry. Finally, feed-through terminations utilizing such protected resistors would be more robust, and less prone to accidental destruction.